Since the immunologically compromised patient frequently develops pneumonias which are difficult to diagnose, aggressive diagnostic techniques are warranted. This study established a method of diagnosing unexplained pulmonary infiltrates in an orderly fashion, utilizing transtracheal bronchial brushing, fibroptic bronchoscopy, or percutaneous lung biopsies when other methods have failed. Forty-nine patients with diffuse or nodular infiltrates have undergone such procedures. Thirteen patients were not clinically infected; eight proved to have pulmonary metastases; four proved to have interstitial fibrosis; and one proved to have a collapsed lobe. Thirty-six patients were clinically presumed infected. Twenty-three were proven to have pulmonary infection. Nineteen of the 23 were etiologically documented by these procedures. Thirteen of the 36 patients had no etiology established by any method, although 10 of these 13 patients were receiving broad spectrum antibiotics at the time of the procedure. The diagnosis included a variety of microrganisms: bacterial, protozoal, and fungal. Significant but not fatal complications occurred and were acceptable risks for the patient population. There was no mortality associated with the procedures, and despite pancytopenia, bleeding was a problem in only two patients. BIBLIOGRAPHIC REFERENCES: Aisner, J., Schimpff, S.C., Bennett, J.E., Young, V.M., Wiernik, P.H.: Aspergillus infections in patients with cancer: In association with fire-proofing materials in a new hospital. JAMA 235:411-412, 1976.